Embodiments of the present specification relates to phase-change thermal interfaces, and more particularly to reusable phase-change thermal interface structures.
Typically, performance of an electronic system, such as electronic circuits is limited by the amount of heat dissipated by the electronic system. The amount of heat dissipated by the electronic system is a surrogate for the maximum junction temperature that the electronic system is allowed to experience. In one example, the electronic system includes a circuit card assembly mounted on a chassis. The circuit card assembly includes a printed circuit board having one or more chips or other surface mounted components. In operation, the one or more chips and/or the surface mounted components employed in the circuit card assembly generate heat and may be referred to as heat sources. For the individual heat sources as well as the electronic system as a whole to function consistently, it is desirable to actively dissipate the heat generated by these heat sources. Sometimes, the heat generated by the heat sources may be dissipated away from the heat sources via a conduction mechanism. Heat spreading structures or heat sinks may be present in the electronic system to facilitate heat dissipation through the conduction mechanism. For example, in the case of the circuit card assembly, the heat sinks may be disposed on the chassis to facilitate heat dissipation using the conduction mechanism. As will be appreciated, in the case of heat dissipation by the conduction mechanism, it is desirable to minimize cavities, crevices or any other surface irregularities that may otherwise be present between mating surfaces of the heat sources and the heat sinks to reduce resistance in a thermal conduction path. For example, in the case of the circuit card assembly, it is desirable to reduce or eliminate the surface irregularities between the chips and the heat sinks.
Generally, thermal interface materials are used in the electronic system to provide the thermal conduction path of low thermal resistance at the interfaces between the mating surfaces of the heat sources and the heat sinks. In particular, the thermal interface materials are positioned at the interface of the heat source and heat sink to fill gaps or voids between the two mating surfaces so that the thermal resistance is lowered, thereby allowing the heat to flow away efficiently from the heat source to the heat sink. However, the thermal conductivity of conventional thermal interface materials is extremely poor, as compared to metals. Typically, in an attempt to reduce the surface irregularities, the thermal conductivity may be compromised.
Also, in addition to reducing the thermal contact resistance, it is desirable for the thermal interface materials to reduce stresses resulting from mismatch in coefficient of thermal expansions of the materials of the heat source and the heat sink during temperature cycling in the electronic circuits. In some cases, the thermal interface materials include adhesives, greases, gels, phase change materials, pads, and thermal pastes. In other cases, the thermal interface materials may include a polymer matrix, such as an epoxy or silicone resin, and thermally conductive fillers such as boron nitride, alumina, aluminum, zinc oxide, and silver. In certain other cases, a liquid metal paste may be used as a thermal interface material. When a component employing the liquid metal paste is de-coupled from the electronic system, the liquid metal paste may be cleaned from surfaces of the component and the electronic system by using metal wool containing tin or copper filaments. However, such liquid metal pastes and other existing thermal interface materials are not configured for reusability. By way of example, cleaning the surfaces to remove the thermal interface material that exists in the form of greases, at each dismantling may become cumbersome. Further, in some cases, the thermal interface materials may not be reusable, and may need to be discarded after each use and replaced by a new thermal interface material.